(1) Field of the Invention
The present invention relates to an improvement of a hysteresis magnet coupling preferably used for Roots type pumps, blowers, vacuum pumps and compressors.
(2) Related Art Statement
Hitherto, the hysteresis coupling comprises a magnet plate holding member provided with a magnet plate and a hysteresis plate holding member provided with a hysteresis plate for transmitting a torque without contact to each other such as to transmit a torque under a slipping condition when the torque exceeds a specified limit during rotation. The hysteresis coupling is generally connected to a machine such as a pump to provide a gap of at most 1 mm between the hysteresis plate and the magnet plate without any interrupter therebetween (see FIG. 2).
Conventional pumps are provided with sealing members such as a mechanical seal, oil seal or the like. In particular, when the hysteresis coupling is applied for a pump provided with a number of sealing members, the following problems result:
(1) Power loss of machine since to the seal member is large and frequently exceeds the torque of the coupling so that the machine can not be started when the machine is operated after a long rest term.
(2) When the machine is operated under an overload, a driving shaft and a pump shaft begin to slip and the hysteresis plate is heated by an eddy current occurring in a magnetic field as the amount of the slippage increases. The heat generated at the hysteresis plate is transferred to the magnet plate and causes the reduction of the magnetic force of the magnet, so that a specified transmitting torque can not be maintained.
(3) The heat transferred to the magnet plate from the hysteresis plate is transferred to the seal members through the pump shaft, resulting in seizure of the seal member of the pump shaft and the others by baking.